Understanding Erosional Activity in Himalayan Rivers

The passage discusses the characteristics of Indian drainage systems, dividing rivers into two main groups: Himalayan and Peninsular. It highlights the differences between these two types, particularly focusing on their origin, flow, and the landforms they create through processes like erosion and deposition.

The question specifically asks about the cause of erosional activity in the upper course of a river. Let's examine what the passage says about the upper course of Himalayan rivers:

• Most Himalayan rivers are perennial (have water throughout the year).
• They originate from north of mountain ranges and cut through mountains making gorges in the upper course.
• They have long courses from source to sea.
• "They perform an intensive erosional activity in their upper courses and carry huge loads of silt and sand with their swift current."

This statement directly links intensive erosional activity in the upper course to their "swift current" or great speed.

Analyzing the Options Based on the Passage

1. the presence of flood plains

   The passage states that rivers form meanders, oxbow lakes, levees, and other depositional features in their floodplains in the middle and lower courses as their speed slackens. Flood plains are features associated with deposition, not the intense erosion typical of the upper course.

2. huge loads of silt and sand

   The passage says rivers "perform an intensive erosional activity in their upper courses and carry huge loads of silt and sand with their swift current." This implies that the huge loads of silt and sand are a *result* of the intensive erosional activity driven by the swift current, not the cause of the erosion itself.

3. the great speed of water

   The passage explicitly states that Himalayan rivers "perform an intensive erosional activity in their upper courses and carry huge loads of silt and sand with their swift current." A swift current means the water has great speed. This high speed gives the water significant energy to wear away the river bed and banks, leading to intense erosion.

4. the reduced flow of water

   The passage mentions reduced flow in the channels of Peninsular rivers during the dry season and states that the speed of Himalayan rivers "slackens" in the middle and lower courses, leading to depositional features. Reduced flow or speed is associated with less energy and typically leads to deposition or less erosion, not intense erosion.

Conclusion

Based on the clear statement in the passage, the intensive erosional activity in the upper course of Himalayan rivers is due to the great speed or swift current of the water. The high velocity allows the river to overcome resistance and erode the landforms it flows through.

The final answer is the great speed of water.

Revision Table: River Processes and Courses

Additional Information: River Dynamics

Rivers are dynamic systems that constantly shape the landscape through three main processes: erosion, transport, and deposition.

• Erosion: This is the process by which the river wears away the land. It happens through various mechanisms like abrasion (grinding with sediment), hydraulic action (force of water), solution (dissolving rocks), and attrition (sediments wearing each other down). Erosion is typically most intense in the upper course where the gradient is steep and water speed is high, providing more energy.
• Transport: The eroded material (sediment, silt, sand, gravel) is carried downstream by the river. Sediment transport occurs in different ways: dissolved load, suspended load (fine particles carried within the water), saltation (larger particles bouncing along the bed), and traction (largest particles rolled or dragged along the bed). The amount and size of material transported depend heavily on the water's speed and volume. Swift currents can transport larger and more material.
• Deposition: This occurs when the river loses energy, typically when its speed decreases. The river can no longer carry its load, and the sediment is dropped. Deposition is most prominent in the middle and lower courses where the gradient is gentler, the channel widens, or when the river enters a standing body of water like a lake or the sea. Depositional landforms include flood plains, meanders, levees, deltas, and alluvial fans.

The passage accurately describes these processes, highlighting how the swift current (great speed) in the upper course drives intense erosion, while the slackening speed in the lower course leads to depositional features.